Combustion in a reciprocating internal combustion engine first of all starts at normal flame velocity. When, at full load, the pressure in the combustion chamber reaches the maximum of approximately 60 bar, about 70-90% of the introduced fuel has been consumed. At this point, fuel mixture components which are still unburned and at some distance from the spark plug, can burn at these high pressure and temperature levels by means of self-ignition which is comparable to the mode of combustion occurring in a diesel engine. However, in a diesel engine, self-ignition is desired for initiating combustion while in an externally-ignited combustion engine, self-ignition, which occurs toward the end of the normal combustion process, is not desired.
Self-ignition toward the end of the normal combustion process is generally designated as "knocking". During knocking, the peak pressure, which at the time of the self-ignition is approximately 60 bar, has a rapid pressure jump superimposed on it. The magnitude of the pressure jump depends on the mixture mass combusted by self-ignition. With very weak knocks, it may be a few tenths of a bar and, with very strong knocks, more than 100 bar. Due to this pressure jump, two effects are initiated in the combustion chamber. The first effect is purely acoustic because every combustion chamber represents an acoustic, vibratable construction which can be excited to self-resonance through sufficiently rapid pressure interference. At the speed of sound in the combustion chamber at the time of combustion (just less than about 1000 m/sec ), these self-resonances are in the range of 7000-25,000 Hz, with the lower resonances around 7000 Hz being dominant. These are the audible frequencies which are also known as "ringing."
For a long time, the theory has been that there are various types of knocking, e.g., knocking at low rpms which occurs during acceleration, and knocking at high rpms and higher loads which occurs under conditions of sustained full acceleration. From a thermodynamic point of view, however, both types of knocking are one and the same. The concept of high-velocity knocking merely indicates that the knocking occurs at higher velocities, so that it can no longer be heard due to the noise of the engine. Consequently, there is then the danger that if knocking continues for a sustained period of time, engine damage will result. Acceleration knocking on the other hand only occurs during a few seconds at the time of acceleration and is generally harmless due to its short duration.
In addition to these low-frequency effects, high-frequency effects also occur during knocking generated by self-ignition. These high frequency effects have thus far hardly been investigated. Schlieren photographs of the combustion chamber of the internal combustion engine lead to the suspicion that shock waves are generated.
A shock wave is a steep pressure wave in a closed area whose steepness is caused by the fact that the speed of sound in the shockwave is no longer constant throughout. This is in contrast to purely acoustical effects where it is assumed that the speed of sound will remain temporally and spatially constant and will not change as the result of the minimal changes in pressure. This limitation is obviated in shock waves because the speed of sound becomes greater at higher pressure, i.e., in the area of high pressure, a pressure wave will run more rapidly than in an area of low pressure. This means that a wave originally in sinusoidal form will cause a very steep pressure jump to occur. Such a jump contains very high frequency components, which is in contrast to the purely sinusoidal wave which contains only the frequencies corresponding to the wave-length. Since very steep pressure jumps, being very high-frequency effects, no longer obey the classical laws of acoustics, the expansion velocity can become considerably higher than the speed of sound, and this effect is designated as a shock wave.
According to the current state of technical knowledge, it is simply known that, during knocking, damage is caused to engine parts and that the extent of damage is related to the intensity of the knocking. However, it is not known for certain which physical events occurring in the combustion chamber are responsible for the damage. There have been indications that it is not the acoustical knock vibrations, i.e., the audible lower-frequency knock vibrations, which are responsible for the damage, but rather the damage is exclusively due to the shock waves generated and the high-frequency compressional vibrations which they cause.
U.S. Pat. No. 2,414,457 to Eldridge et al discloses a device for ascertaining rapid pressure changes in the combustion chamber of an internal combustion engine. Such a device allows the measurement of a voltage which is representative of the level, rapidity of change and other characteristic properties of these pressure changes. The measurement is accomplished by means of a rod made of magnetostrictive material consisting of an alloy of about 52% nickel, about 48% iron and small quantities of other materials. The changes of magnetic flow occurring in this rod due to pressure changes are transformed into voltage signals in a coil, and such generated signals may then be processed further.
This device, however, is not suitable for measuring in a precise manner those types of knocking which lead to engine damage in high-performance operation. This may be due to the fact that the magnetostrictive receiver of the known type device has a relatively large receptive surface in relation to the size of the combustion chamber. As a consequence, when high-frequency pressure changes occur and/or shock waves impact on the receptive surface, an integrative effect occurs which does not permit exact measurement of the type, velocity, frequency, amplitude, etc., of the pressure change occurring.
In addition, magnetostrictive receivers of the known type have a relatively short length, such that the relationship of the length to the diameter of the receiver is relatively small. In the above-mentioned patent, this length to diameter relationship has an approximate value of 30, which causes additional non-exactnesses in measurement.